Date, Calendar

1
Date, Calendar DateFormat Objects

• java.util.Date
• a specific instant in time.
• java.util.Calendar
• conversion of Date to integer fields.
• java.text.DateFormat
• format and parse date/time strings.

2
Date

• A Date object represents a specific instant in
  time.
• In old versions of Java the Date object was used
  for lots of things, now Calendar does most of the
  work.
• Most of the methods are deprecated!
• Non-deprecated use includes
• conversion to/from the number of milliseconds
  since Jan 1, 1970.
• Comparison of dates (after, before, equals, ...)

3
Print Current Date/Time

• import java.util.
• class DatePlay
• static public void main(String args)
• Date d new Date()
• System.out.println("The date is " d)

gt java DatePlay The date is Sat Mar 01
204616 EST 2003
4
Calendar

• abstract class!
• GregorianCalendar is the only standard derived
  class included (1.4).
• Object Factory
• static Calendar getInstance()
• Locale and Timezone Objects
• The idea is use a Calendar object to take care of
  date/time handling details in a manner that
  supports the conventions/calendar type of the
  user.

5
Calendar Object

• Can set/get integer values for day, month, year,
  hour, minute, second, day of the week, etc.
• Two (overloaded) methods
• set(int field, int value)
• get(int field)
• You have to know what the value of field should
  be to get what you want!
• The Calendar class provides some static int
  fields that make this possible.

6
Calendar Fields

• Calendar.DATE Calendar.MONTH
• Calendar.YEAR Calendar.DAY_OF_WEEK
• Calendar.DAY_OF_MONTH Calendar.DAY_OF_YEAR
• Calendar.HOUR Calendar.MINUTE
• Calendar.SECOND Calendar.MILLISECOND
• There are lots more!

7
Calendar Example

• import java.util.
• class CalPlay
• static public void main(String args)
• Calendar c Calendar.getInstance()
• System.out.println("Today is "
• (c.get(Calendar.MONTH)1) "/"
• c.get(Calendar.DATE) "/"
• c.get(Calendar.YEAR))

8
Another Example

• class CalPlay
• final static String DAYS
  "Sunday","Monday",
• "Tuesday", "Wednesday", "Thursday",
• "Friday", "Saturday"
• static public void main(String args)
• Calendar c Calendar.getInstance()
• System.out.println("Today is " dayName(c))
• static String dayName( Calendar c)
• return(DAYSc.get(Calendar.DAY_OF_WEEK)-1
  )

9
Even Better Example

• tomorrow.java and bettertomorrow.java
• prints out the date tomorrow.
• needs to watch for new month, year, etc.
• Does this for any date entered as month day year
  on the command line (as three integers).

gt java tomorrow 12 31 2001 Tomorrow is Tuesday
January 1, 2002
10
Homework

• Write a program that prints out the (single
  month) calendar for any month.
• Just like the Unix cal command.

gt java cal 3 2003 March 2003 Su Mo Tu We Th Fr
Sa 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31
gt java cal 1 2025 January 2025 Su Mo Tu We Th
Fr Sa 1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
26 27 28 29 30 31
11
DateFormat

• Another abstract class
• Factory method
• static DateFormat getDateInstance()
• Uses a Calendar to format dates.
• associated with a Locale
• A number of pre-defined formats (styles)
• short, medium, long, full
• DateFormat.SHORT, DateFormat.MEDIUM, ...

12
DateFormat Example
3/1/03 Mar 1, 2003 March 1, 2003 Saturday,
March 1, 2003

• public static void main(String args)
• Date now new Date()
• // try with default Locale
• showDate(DateFormat.SHORT,now)
• showDate(DateFormat.MEDIUM,now)
• showDate(DateFormat.LONG,now)
• showDate(DateFormat.FULL,now)
• static void showDate( int style, Date d)
• System.out.println(
• DateFormat.getDateInstance(style).forma
  t(d))

13
DateFormat Exemple (Locale for France)
01/03/03 1 mars 2003 1 mars 2003 samedi 1
mars 2003

• public static void main(String args)
• Date now new Date()
• showDate(DateFormat.SHORT,now)
• showDate(DateFormat.MEDIUM,now)
• showDate(DateFormat.LONG,now)
• showDate(DateFormat.FULL,now)
• static void showDate( int style, Date d)
• System.out.println(
• DateFormat.getDateInstance(style,
• Locale.FRANCE).format(d)
  )